1. Technical Field of the Invention
The present disclosure is related generally to fluid flow control and electro-hydraulic/electro-pneumatic systems, and more particularly, to a valve positioner including a failsafe that maintains the position of the valve to that of a pre-failure state.
2. Description of the Related Art
A control valve regulates a flowing fluid, such as gas, steam, water, or chemical compounds by opening and closing a passageway, through which the fluid flows, with a valve element. The subject flowing fluid is generally referred to as the process. An actuator, in turn, provides the motive force to open and close the valve element. Pneumatic or hydraulic energy is converted by the actuator to rotational or linear motion, depending on the configuration of the valve element.
Typically, pneumatic systems are utilized for valve actuators due to several distinct advantages. For instance, air, rather than fluids such as oil, is exhausted into the atmosphere, and compressed air is better able to absorb excess pressure and pressure spikes. There are other peripheral advantages such as fewer maintenance requirements.
A conventional pneumatic actuator is comprised of a piston sealed within a cylinder, and the piston including a connecting rod that is mechanically coupled to the valve element. Compressed air is forced into and out of the cylinder to move the connecting rod. In a single-acting actuator, the compressed air is taken in and exhausted from one end of the cylinder and is opposed by a range spring, while in a double-acting actuator, air is taken in one end of the cylinder while simultaneously exhausting it out of the opposing end.
Precise and accurate control of the valve actuator, and hence the valve element, can be achieved with a positioner device coupled thereto. Pneumatic valve positioners, which can cooperate with aforementioned pneumatic actuators, are well known in the art. The proportional movement of the actuator is accomplished by the movement of compressed air into and out of the actuator piston, as indicated above. More particularly, valve positioners incorporate a spool (or other devices) that either rotates or slides axially in a housing the port the flow of compressed air to the actuator or to one or more exhaust ports.
In further detail, an electrical control circuit provides a variable current signal to the positioner device that proportionally corresponds to particular states of the actuator and hence a particular position of the control valve. The electrical control circuit and the electrical current signals generated thereby may be part of a broader process managed by a distributed control system (DCS). Generally, the electrical current varies between 4 milliamps (mA) and 20 mA according to industry-wide standards; at 4 mA the valve positioner may fully open the valve element, while at 20 mA the valve positioner may fully close the valve element. The positioner compares the received electrical signal to the current position of the actuator, and if there is a difference, the actuator is moved accordingly until the correct position is reached.
There are a number of operational conditions or exceptions under which it becomes necessary to “freeze” in place the last position of the actuator. These include the complete loss of power to the positioner or other such failure therein, failure in the distributed control system, a wire carrying the actuator signal being cut, and so forth.
Various solutions for such “fail freeze” functions have been developed, though each one is deficient in one or more regards. One involves the use of an external component to monitor the electrical current signal, and driving a solenoid valve upon detection of a failure condition. This tends to be an expensive proposition, however, since a safe external power source is required, along with specialized components that monitors the electrical current such as a current threshold switch and controls the power to the solenoid. Additionally, a further wiring and junction box will be required. Overall, the increased complexity of this solution makes it particularly unsuitable (e.g., too expensive) for hazardous environments. Another solution involves the use of a positioner with normally closed on/off valves. This is also inadequate because the flow capacity of such positioners is typically so low that boosters are necessary to meet the specified stroking time. Furthermore, any leakage from the boosters essentially nullifies the freezing action. Yet another solution involves a pneumatic positioner with a separate fail-freeze electro-pneumatic I/P converter. Again, this solution has proven deficient, as the separate positioner has a slow response time of around six (6) seconds, such that stroking the actuator within the required limits is not possible.
Accordingly, there is a need in the art for an improved valve positioner with a failsafe that maintains the position of the valve to that of a pre-failure state. Moreover, this is a need in the art for a valve positioner that includes a fail-freeze function powered from the electrical current signal loop thereto without an external source. There is also a need for valve positioners with a fail-freeze function that are intrinsically safe.